Those of ordinary skill in the art will be familiar with a very wide variety of so-called centralizers employed in the processes of oil and gas exploration and production to maintain a segment of tubing (“tubular”) in a substantially centralized longitudinal position relative to a surrounding barrier, e.g., a borehole wall, well casing, or a larger tubular). The desire to keep tubulars centralized, and the benefits and advantages of using devices or structures to maintain centralization, are well known to those of ordinary skill in the art.
Among the many different types of centralizers that are presently known, a subset of them can be roughly categorized into a class of so-called “bow spring” centralizers. Bow-spring centralizers are characterized as such due to their having at least one, and more common, a plurality of bow-spring elements adapted to press against an outer barrier or wall and exert a radial inward force on the tubular, such that the tubular tends to be deflected away from the wall. The class of bow-spring centralizers is generally distinguished from another class of centralizers having radially oriented flange-like features adapted to deflect the tubular radially inward toward a central position within a borehole or other tubular enclosure.
Typically, a bow-spring centralizer has a plurality of bow-springs arranged concentrically around a tubular and held at each end by a circumferential collar adapted to be installed around the tubular to be centralized. Each centralizer extends radially outward from the outer surface of the tubular to press against a sidewall thereby exerting a radially-directed inward force upon the tubular. The net effect of the plurality of centralizers is that the tubular is effectively maintained in a relatively central position within the surrounding sidewall or structure.
A known advantage of bow-spring centralizers is that so long as at least one of the end collars is free to slide longitudinally along the tubular it surrounds, the centralizer is capable of being compressed inwardly, so as to be able to progress through passages that are narrower than the diameter of the centralizer in an uncompressed state. Provided that such a centralizer is fashioned from a material such as spring steel or the like that will return to a fully uncompressed form when not compressed by outer forces, the centralizer can adapt to conditions, such as within a borehole, in which the path taken by the centralizer is of varying diameter along its length.
The number of prior art examples of bow-spring centralizers is so large, and the general concept of operation and use of bow-spring centralizers is so well known, that no particular prior art example would necessarily stand as “exemplary” of the entire class of bow-spring centralizers.
Conventional bow-spring centralizers are typically provided with a plurality of bow-springs (e.g., four or more), equally spaced around the circumference of a tubular and held or otherwise secured at each end to a cylindrical collar adapted to fit around the outer circumference of the tubular to be centralized. In many prior art designs, each bow spring is a separate element, and a mechanical means is required to attach each end of each bow-spring to an end collar. Innumerable variations of such bow-spring centralizers have been proposed in the prior art. The bow-springs may be mechanically interlocked with the end collars, as proposed by U.S. Pat. No. 6,871,706 to Hennesey, entitled “Casing Centralizer,” or the bow-springs may be affixed to end collars by means welding and/or with 3 connection pins, screws, rivets, or the like. Once again, innumerable examples of this type of bow-spring centralizer exist in the prior art. See, for example, U.S. Pat. No. 5,575,333 to Lirette et al., entitled “Centralizer.”
The use of mechanical means for interconnecting a centralizer's bow springs with its end collars has proven to be reasonably effective in the oil and gas industry. However, it has long been realized that it is desirable to provide a centralizer design that has minimal impact on the overall outer diameter of the centralized tubular, in order for the tubular to travel through passageways which may constrict at certain points to a diameter only marginally larger than the tubular itself.
Thus, for example, it is been recognized that any means of connecting a bow-spring to its end collars that tends to project radially outward from the centralized tubular to any appreciable extent is generally undesirable. Any such feature of a centralizer will tend to increase frictional forces on the tubular's travel. This is addressed, for example, in U.S. Pat. No. 6,679,325 to Buytaert, entitled “Minimum Clearance Bow-Spring Centralizer.”
Furthermore, many of the means of connecting bow-springs to respective end collars providing for the least radial expanse of the tubular/centralizer combination are susceptible to mechanical failures where bow-springs can become detached from their end collars and hence rendered incapable of functioning as intended. This is true, for example, of designs in which the bow springs are welded at each end to the end collars, as is the case in many prior art implementations.
To avoid the necessity of mechanically fastening bow springs to the end collars, it has been proposed in the prior art to form a centralizer out of a flat sheet of steel, with apertures being formed therein to define end collar regions and bow-spring regions. The flat sheet is then rolled into a substantially cylindrical form, with respective sides of the flat sheet coming together to form a longitudinal seam in the resulting cylindrical centralizer. This is shown, for example, in U.S. Pat. No. 6,997,254 to Jenner, entitled “Method of Making a Centering Device and Centering Device Formed by That Method.”
The approach proposed in the Jenner '254 patent may be deemed less than optimal, inasmuch as it merely substitutes the need for mechanical fixation between opposing edges for the need for mechanical fixation of the bow-springs to the end collars. The Jenner '254 patent suggests that the respective edges of the rolled structure can be mechanically coupled by means of hinge pins or interlocking finger portions. In either case, this mechanical coupling may be susceptible to failure, and the presence of protruding features is not avoided. Moreover, the Jenner '254 approach involves the additional fabrication step(s) and associated tooling that would be necessary to roll the initially flat sheet(s) of steel to form a cylindrical centralizer.